Experiments designed to investigate the nature of oxidation- reduction reactions of copper with respiratory cytochromes and related model compounds have been initiated. The principle objectives of the study are to assess the potential role of pi-unsaturated ligands in mediating electron transfer between metal ion redox centers in biological molecules and to elucidate electron transfer pathways in cytochrome oxidase. Impetus for the study derives from the recent structural characterization of heme a and our own research on the spectra and electron transfer reactions of cobalt(III)-copper(I) dimers containing copper pi-coordinated to cobalt alkenoate ligands. Specific studies encompassed by the proposal are investigation of: (1) Cu(I) binding to heme a and the kinetic properties of Cu(I)-heme solutions, (2) the effect of increasing pi-unsaturation in organic ligands upon Cu(I) reduction of metalloporphyrins and other transition metal oxidants, (3) internal electron transfer reactions between redox components of cytochrome c oxidase with emphasis upon detecting electron transfer involving copper and evidence for cooperative interactions between metal centers, (4) the effect of Cu(I) coordination in promoting electron transfer between redox ions by pi-overlap pathways as a means of identifying transfer mechanisms (e.g., resonance exchange, superexchange).